Smoke+Fire Simulation Fire Not Rendering
The Color output of the flame attribute isn't what you need. Instead connect the Fac output to the ramp. The Fac contains data about the flame intensity, and connecting it to the ramp, will define the color based on the intensity.

Moreover, your intensity multiplier is set quite low. This is not an error, but it may get you very low visibility on the flames.

This image shows the defaults that get set when using Object -> Quick Effects -> Quick Smoke